This invention relates to the field of chemical vapor deposition (CVD), and more particularly to CVD reactors of the type featuring a gas inlet manifold or face plate in close facing relationship to a substrate upon which deposition is desired.
CVD, in its many forms, is a process which may used to apply a thin layer or film of material to a substrate, typically a semiconductor wafer, by introduction of reactant gasses in the presence of applied heat. The wafer is typically held on a flat susceptor or other heater element. Common deposited materials include silicon nitride, silicon oxide, silicon oxynitride and/or tungsten silicide. Some reactors utilize a plasma of the reactant gasses in a plasma enhanced CVD process (PECVD). Common means of forming the plasma used in PECVD reactors are radio frequency (RF) excitation. In common constructions, both PECVD and basic CVD reactors feature a flat plate distribution manifold or "face plate" in an operative position in close facing relationship to the face of the substrate onto which the film is to be deposited. As is shown in a most schematic form in FIG. 1, the manifold 10 typically has flat upstream and downstream faces 12 and 14 with a central array of holes 16 extending therebetween and covering an area corresponding to the plan area of the substrate 18. Reactant gas 20 flows through the holes to the substrate. In RF reactors, the manifold serves as an RF electrode. In some CVD reactors, one manifold configuration involves a hole array wherein the holes are formed as narrow straight bores arranged along the circumferences of circles of incrementally greater radius starting in the center of the face plate and spanning a region from the center to the a radial location approximately the same as the radius of the substrate. In such a configuration, the majority of the surface area of the upstream and downstream faces of the manifold is left intact and unaffected by the holes (FIG. 2).
In one particular design for a manifold used in an RF reactor, shown in U.S. Pat. No. 4,854,263, issued Aug. 8, 1989, the disclosure of which is incorporated herein by reference, the straight bores are replaced with holes having an outlet diameter greater than their inlet diameter so as to increase the dissociation and reactivity of reactant gas passing through the manifold. To maximize hole density, the holes are arranged in an hexagonal closed packed array.
During the CVD process, deposition occurs not simply upon the target face of the wafer but, especially, on any hot surface such as a heater 19 or a susceptor. The growth of such unwanted deposit layers can cause any of a number of problems. To keep unwanted deposits under control, a reactive plasma cleaning may be performed, often at predetermined intervals during a process run, such as after every n.sup.th wafer is processed. Typical cleaning processes include the introduction of a cleaning gas and the application of heat. As with deposition, the heat may come from a number of sources. Typical cleaning gases are compounds of fluorine such as chlorine trifluoride (ClF.sub.3), or perfluorocompounds such as nitrogen trifluoride (NF.sub.3).
Although an inherent purpose of cleaning is to maintain the performance of the reactor, a correlation has been observed between reactor cleaning and degradation in the consistency and the uniformity of deposition. In particular, it is believed that periodic cleaning of the reactor with a gas such as ClF.sub.3 may result in the formation of aluminum fluoride (AF) deposits on the manifold. Specifically, the gas flows through the manifold along the same path as reactant gas 20 of FIG. 1, and then comes into contact with the hot heater 19 (there typically being no wafer 18 in the reactor during cleaning). With the heater formed of a substance such as aluminum nitride (AlN), fluorine radicals are believed to react with the heater to produce AlF.sub.3, which joins the radial outward flow 22 of gas in the space between the heater and manifold. The flow 22 shown in FIG. 1 is highly generalized and schematic. An actual flow may involve a variety of flow structures which can provide gas transport to and between the surfaces of the heater (when the wafer is removed) and face plate.
Because the manifold is kept at a relatively lower temperature than the heater, the AlF.sub.3 molecules are believed to preferentially deposit on the manifold. These deposits are believed to increase the emissivity of the manifold and, thereby, to reduce the temperature of the wafers in subsequent processing. This has led to observed reductions in film thickness and resistivity in wafers processed after cleaning. Additionally, the altered emissivity is believed to be associated with a lack of deposition uniformity across the surface of each individual wafer processed in the reactor. In particular, a reduced resistivity has been observed near the wafer perimeter relative to resistivity near the wafer center.
It is believed that deposits on the portion 24 of the manifold immediately opposite the wafer, i.e. in the central circular region of the manifold of the same radius as the wafer, have significant effect on the wafer and process. However, deposits on the portion 25 of the manifold immediately radially beyond this region also have significant effect, especially upon peripheral portions of the wafer. In particular, deposits on the portion of the manifold immediately beyond the perimeter of the wafer may be involved in the lack of uniformity of deposition on a given wafer. The peripheral portion of the wafer is still in sufficient proximity to the portion 25 of the manifold immediately laterally beyond the wafer so that an emissivity increase along this portion 25 of the manifold will decrease wafer temperature near the wafer periphery.
Accordingly, it is desirable to alleviate or reduce the separate and combined effects of deposits in both the central and peripheral regions of the manifold.